Elephants can use scent to distinguish more from less

An elephant nose is nothing to sneeze at.

Rippling with 150,000 individual units of muscle and tendon, the trunks of these behemoths are veritable Swiss army knives of functionality, deployed for everything from feeding to fighting. Each trunk can store up to two gallons of water, and has the strength to lift hundreds of pounds and the dexterity to crack a single nut from its shell.

But these supersized snoots are constantly serving up surprises—and a group of researchers has now discovered that they’re discerning enough to distinguish more from less. Their study, published today in the journal PNAS, suggests elephants can use scent alone to differentiate between quantities of sunflower seeds that might flummox even the most hawk-eyed of humans.

“This is a very impressive paper,” says Ashadee Kay Miller, a biologist at the University of the Witwatersrand in South Africa who was not involved in the study. “I think they’ve gone through a lot of effort to show that what they’ve found is genuine.”

While it’s still not clear exactly how elephants sniff out larger quantities, the scientists’ findings reinforce the idea that elephants depend heavily on odors to guide their behavior—and may help scientists study their cognitive capabilities in the future.

Olfaction, or sense of smell, remains vastly understudied compared to senses humans rely on more, like vision and hearing. People are naturally biased toward studying what we know—and a lot of what we know comes from what we see. But elephants are among the many animals for which sight is a second-class sense.

Studies have shown that elephants boast more than 2,000 genes coding for olfactory receptors—the most of any mammal tested to date (for comparison, humans have only 400). Elephant noses are sensitive enough to detect TNT, and distinguish between tribes of humans that have traditionally treated them well and groups that have acted aggressively. They even have the precision to discriminate between structurally similar molecules that are mirror images of each other—a subtle difference that would stump the schnozzes of humans and other primates.

But these stunning discoveries likely only scratch the surface, and the true limits of elephants’ olfactory prowess remain mysterious.

To fill in some of the gaps, a team of researchers led by Joshua Plotnik, a comparative psychologist at Hunter College, City University of New York, decided to see whether six captive Asian elephants (Elephas maximus) could use odors alone to make quantitative judgments.

After training their subjects to seek out containers with larger amounts of food, Plotnik and his team presented each elephant with a pair of locked, opaque buckets containing two different quantities of sunflower seeds. While the elephants couldn’t see the seeds, they could still catch a whiff of each container’s contents through perforations in its lid. After a brief period of investigation, the buckets were unlocked so the elephants could indicate their choice by removing the lid from one container.

Across 11 different ratios of seeds, the elephants were, on average, able to pick out the larger helpings of food. While the elephants did their best work when the disparities between quantities of food were larger—for instance, when selecting 24 grams of seeds over 4 grams (roughly 180 and 30 seeds, respectively)—their performance steadily diminished as the differences between seed portions shrank. Overall, however, “they did pretty well,” Plotnik says. “A couple of the elephants were even able to tell [approximately] 180 seeds from 150, and that’s remarkable.”

Ensuring that the elephants were using only scent-based cues, however, required some creativity, in part because smells tend to linger. But when the researchers replaced their plastic buckets for metal receptacles that might be less likely to retain odors, or repeated their tests with containers that had previously held vastly different numbers of seeds, the results still held true.

The team also ruled out the possibility that their study subjects were picking up on inadvertent cues from the researchers by adding an experiment in which they hid the quantities of seeds from human and elephant alike. In a final trial, the team even positioned the buckets in a way that equalized the height of the top of each seed pile to keep the elephants from picking the portion closer to their trunks. In spite of all these changes, the elephants continued to home in on the more generous portions.

One condition did strip the elephants of their ability to detect the aroma of abundance: swapping out the perforated lids for solid ones that scents couldn’t pass through. This, Plotnik says, hints strongly at the incriminating culprit being smell alone.

“I’m really glad to see a study like this done,” says Caitlin O’Connell, a biologist and elephant expert at Stanford University who was not involved in the study. “I know how difficult these studies are, but these experiments are really nicely set up. They leave little left to be questioned about whether what they see [is real].”

Because the elephants in the study weren’t free-living, it’s not yet clear how these behaviors translate to wild populations, or how they’d benefit elephants in social or ecological contexts, says Marcela Benítez, a behavioral biologist at Georgia State University who was not involved in the study. But smell can have a lot of advantages over vision. “When you have visual stimuli, it’s only as good as what’s in front of you,” she says. “It doesn’t travel well. But something like scent can remain...it has a temporal element you don’t get with vision.”

And given how smell-oriented elephants are in navigating their surroundings, it’s likely they’re putting these appendages to good use in daily life, O’Connell says. Quantitative trunks could come in handy for locating rich caches of food, or keeping track of a herd. They could also help elephants identify potential mates or avoid conflicts with competitors. “It’s critical for elephants to understand their world through smell,” she says. “For them to be able to distinguish quantity makes a whole lot of sense.”

What exactly “more” or “less” smells like at the cellular level, and how this information gets processed in the brain, still aren’t clear. (Though, based on the study’s results, it’s probably unlikely that elephants can sniff a container and tell the exact number of seeds inside.) Answering these questions will require more research—and, given our own limitations in the odor department, a hefty dose of creativity. “[Understanding how to smell quantity] might be like describing colors to someone who’s never seen before,” Miller says.

Understanding this sensory skill, however, could help preserve a species in decline. Fewer than 40,000 Asian elephants remain in the wild—and “we’re running out of time to save them,” Plotnik says. “They’re remarkably intelligent animals we still know very little about.”

But conservation will be difficult without clearer insights into how these animals see—and smell—the world around them. “If you want to really understand how smart an animal is, you have to put yourself in their shoes,” Plotnik says. “What this study tells us is, if you want to do that with an elephant, you’re going to need a trunk.”